The use of surfactants in micellar enhanced ultrafiltration (MEUF) has attracted considerable attention since Dunn et al1,2 first named the process and demonstrated the technique by recovering 4-tert-butylphenol from aqueous solutions. With the increase in public concern about environmental pollution and energy consumption, the classical separation techniques are becoming insufficient to meet strict environmental requirements. Since the toxic organic molecules that become associated with surfactant micelles have been found to be more effectively prevented from passing through the pores in membranes in the MEUF process (i.e., retained by the membrane) than the molecules not included in micelles, surfactants have been used effectively for enhancing the removal of organic contaminants from aqueous solutions.3 
The performance of various surfactants for removal of phenols from aqueous solutions in the MEUF process has been studied. Using phenol as a model compound in these studies, the effectiveness of surfactants is often represented as the rejection ratio of the phenol,4 defined as the percentage of phenol that is retained by the membrane in the retentate as shown in the equation below.
                    R        =                              (                          1              -                                                                    [                    P                    ]                                    m                                                  [                  P                  ]                                                      )                    ×          100          ⁢          %                                    (        1        )            where “P” stands for phenol and the subscript “m” stands for micellar phase.
Cationic surfactants have been studied most intensively because of their positive charge, which attracts ionized phenol molecules (phenolates) electrostatically. Alkyl trimethylammonium salts2˜10 and cetylpyridinium chloride (CPC)11˜14 have been reported to have high effectiveness for removal of phenol from aqueous solutions. The rejection ratio of phenol in an MEUF process containing cationic surfactant micelles increases dramatically with pH. For comparison, anionic sodium dodecyl benzene sulfonate3 and sodium dodecyl sulfates4,5,7,9,11,15 were also studied; relatively low rejection ratios were reported, and the rejection ratio of phenol was observed to decrease with pH.
A drawback of MEUF is that monomeric surfactant molecules not retained by the membranes can recontaminate the otherwise purified permeate water. The monomer concentration is mainly determined by the critical micellar concentration (CMC) of the surfactant used. Generally, ionic surfactants have a higher CMC than do nonionic surfactants due to electrostatic repulsion; therefore, nonionic surfactants are preferred in terms of minimizing recontamination. Nonionic nonyl phenol ethoxylates3,16, polyoxyethyleneglycol alkylether17,18 and alkyl polyglucoside4 have been studied. The performances of these nonionic surfactants have shown the least dependence on pH compared with cationic and anionic species. In addition, block copolymer has also been studied for MEUF, and the rejection ratio of phenol has been reported to be approximately 75%19. Since the rejection ratio of phenol depends on the concentration and the composition, an equilibrium constant11 has been defined in order to compare the performances of various surfactants.
                              K          eq                =                                            [              P              ]                        m                                                              [                P                ]                            a                        ⁢                          (                                                [                  S                  ]                                -                cmc                            )                                                          (        2        )            where “S” stands for surfactant. The subscript “a” stand for aqueous phase. “[S]-cmc” is the concentration of the surfactant which contributes to the surfactant micelles.
Most of the studies of MEUF focus on the flux of permeate and the rejection ratio of phenols under various conditions. However, the mechanism of the association of phenol with surfactant micelles remains unclear. The interactions between surfactants and phenols, the distribution of phenols in aqueous and micellar phases, and the effects of phenols on the surfactant micellization are not fully understood.
Ultrafiltration techniques has also been used for separation of surfactant monomers from micelles for understanding of the micellization behaviors of surfactants20˜25. In the ultrafiltration process, surfactant micelles are retained in the bulk solution (retentate), while solvent molecules and surfactant monomers pass through the pores in the membrane (permeate). By measuring the surfactant concentration in the permeate, the concentration of surfactant monomers is determined and related to the micellization behavior of the surfactant molecules. Huang, et al,23 have reported abnormal changes in monomer concentration using ultrafiltration and suggested coexistence of different micelle species.
When phenol solubilizes in micelles, it may affect the size and shape of the micelles. It has been reported10 that phenol solubilizes in the palisade layer and not in the micellar core, and the hydrophilic heads of surfactants also affect the solubilization of phenol. Kim et al17 have reported a decrease in rejection of phenol in response to an increase in the number of oxyethylene groups in the surfactant polyoxyethyleneglycol alkylether CnEm, which suggests that hydrophobicity of the surfactant is favored for the solubilization of phenol.